Thermocoup le

Mechanical Engineering Department

Selection of an Instrument for a Need….

by:-by:- Shyamakant sharan(0105me121102)Shyamakant sharan(0105me121102) Sonu kumar patel(0105me121104)Sonu kumar patel(0105me121104)

Thermocouples

A thermocouple is a temperature-measuring device consisting of two dissimilar conductors that contact each other at one or more spots. It produces a voltage when the temperature of one of the spots differs from the reference temperature at other parts of the circuit. 

Misconception in Thermoelectricity

The EMF generated by the Seebeck effect is due to the temperature gradient along the wire.

The EMF is not generated at the junction between two dissimilar wires.

Principle of operation

Sheath options

Thermocouples: Seebeck effect

The Seebeck effect is the conversion of thermal energy/temperature differences directly into electrical energy or electricity.

This effect measures the ease at which excess electrons will circulate in an electrical circuit under the influence of thermal difference.

The change in the voltage is proportional to the temperature difference between the junctions when the ends are connected to form a loop.

A thermoelectric circuit composed of materials of different Seebeck coefficient (p-doped and n-doped semiconductors), configured as athermoelectric generator.

If the load resistor at the bottom is replaced with avoltmeter the circuit then functions as a temperature-sensing thermocouple.

Seebeck Effect

Circuit construction

Standard Calibration Setup

Ice-point

The result of the calibration is a table of EMF versus T values. The integral is never directly evaluated. Instead a polynomial curve fit to the calibration data gives:

( ) njnjjjj TbTbTbbTFE ++++== .....2

2100

Material EMF versus Temperature

With reference tothe characteristicsof pure Platinum

emf

Temperature

Chromel

Iron

Copper

Platinum-Rhodium

Alumel

Constantan

Thermocouple temperature vs.Voltage graph

Seebeck coefficient vs. Temperature

applications

1.Steel industry2.Gas appliance safety3.Thermopile radiation sensors4.Manufacturing 5.Power production6.Thermoelectric cooling7.Process plants8.Thermocouple as vacuum gauge

A thermocouple (the right most tube) inside the burner assembly of a water heater.

Thermocouple connection in gas appliances. The end ball (contact) on the left is insulated from the fitting by an insulating washer. The thermocouple line consists of copper wire,

insulator and outer metal (usually copper) sheath which is also used as ground.

Type

Thermocouple insulation

Advantages

The end